Various polymeric materials have been widely used over the past 60 years for molding toothbrushes, tool handles, windshield scrapers, steering wheels, hair brushes, cutlery, eyeglass frames and the like. In many of these applications, the molded part must be clear, tough, impact resistant, stress crack resistant, hydrolysis resistant as well as having a pleasing feel and appearance.
Plasticized cellulose acetate propionate (CAP) compositions have been used successfully in the past for toothbrush handles. Such compositions have good clarity, sparkle and overall appearance. However, design changes in toothbrush handles to increase the bristle density has led to cracking in certain brushes, The cracks which occur during bristle insertion are a result of insufficient weld-line strength. Increased plasticizer concentrations improve the weld-line strength but this leads to decreased stiffness which can result in inadequate bristle retention.
Certain rigid polyurethane materials have been evaluated in this application but this polymer is difficult to mold, and the urethane linkages in the polymer chain can hydrolyze in the presence of moisture during molding.
Polyester materials such as poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) have many desirable properties for molded parts but these polymers are readily crystallizable and provide hazy or opaque objects when molded in thick parts. Modification of PET polymers with high levels of glycol components other than ethylene glycol provide clear, tough molded parts but they tend to stress crack in the presence of certain toothpaste solutions containing mint oil.
For example, U.S. Pat. No. 2,901,466 (1959) assigned to Eastman Kodak Company describes a wide range of linear polyesters and polyesteramides derived from 1,4-cyclohexanedimethanol (CHDM). Many of the compositions are readily crystallizable and molded parts are hazy or opaque. Thus, they are not suitable for clear, molded objects.
There is a need in the art, therefore, for molding compositions which have visual clarity and which have improved molding and physical property requirements.